1. Field of the Invention
The present invention relates to a method and apparatus for controlling a lifting magnet of a materials handling machine for which the source of DC electrical power is a DC generator. It finds particular application in conjunction with lifting magnets used on crawlers in the scrap metal industries.
2. Prior Art
Lifting magnets are commonly attached to crawler booms to load, unload, and otherwise move scrap steel and other ferrous metals.
While lifting magnets have been in common use for many years, the systems used to control these lifting magnets remain relatively primitive. During the “Lift”, a DC current energizes the lifting magnet in order to attract and retain the magnetic materials to be displaced. At the end of the “Lift”, when the materials need to be separated from the lifting magnet, most of the controllers automatically apply a reversed voltage across the lifting magnet for a short period of time to allow the consequently reversed current to reach a fraction of the “Lift” current. This phase is known as the “Drop” phase, during which a magnetic field in the lifting magnet of the same magnitude but in an opposite direction of the residual magnetic field is produced that the two fields cancel each other. When the lifting magnet is free of residual magnetic field, all scrap metal detaches freely from the lifting magnet. This is known as a “Clean Drop”.
Some known control systems operate to selectively open and close contacts that, when closed, complete a “Lift” or “Drop” circuit between the DC generator and the lifting magnet. At the end of the “Lift”, which is called the “discharge” and at the end of the “Drop”, which is called the “secondary discharge”, these systems generally use either a resistor or a varistor to discharge the lifting magnet's energy. The higher the resistor's resistance value or varistor breakdown voltage, the faster the lifting magnet discharges, but also the higher the voltage spike across the lifting magnet. High voltage spikes cause arcing between the contacts. In addition, fast rising voltage spikes also eventually wear out the DC generator collector and its winding insulation, the lifting magnet insulation, and the insulation of the cables connected to the lifting magnet and the generator. To withstand these voltage spikes, generally in the magnitude of 750 V DC with systems using DC generators rated 240 V DC, the lifting magnet, cables, and the control system contacts and other components must be constructed of more expensive materials, and must also be made larger in size. These systems waste lifting magnet's energy. Lifting magnet's energy is transformed into heat, dissipated through a voltage suppressor or resistor bank. This results in poor system efficiency and oversized components to dissipate the heat.
To avoid these issues, some other known control systems connect directly to DC generator excitation shunt field. They eliminate arcing across contacts and minimize voltage spikes in the lifting magnet circuit but at the expense of a slower response time, caused by the induced DC generator time constant.